The invention relates generally to optical scanners, and in particular to the light collection optics thereof.
It is a common concern of designers of optical scanners, particularly although not only designers of portable bar code scanners, to ensure that neither the scanner size nor its weight becomes too large. The size constraints have meant that portable laser bar code scanners normally use non retro-reflective scanning techniques, thereby eliminating the need to use large scanning mirrors. However, the field of view of a non-retro-reflective scanner is large and fixed, resulting in a relatively high level of background noise and noise from laser speckle, thereby causing scanner performance to suffer. Additionally, small scanning mirrors may be used only in certain applications such as hand-held omni-directional scanners, integrated scanners, or scanners in which the scanning element is mounted to a single chip. In other applications, it may be technically preferable to use much larger mirrors but small photodetectors.
There is a need to make retro-reflective scanning technology more widely available, particularly for portable laser bar code scanners, thereby obviating at least some of the difficulties which are associated with non-retro-reflective scanners.
One approach for dealing with this problem is disclosed in U.S. Pat. No. 5,332,892, commonly assigned with the present application. This discloses a system in which non-conventional optics and sensing elements are used to expand the scanner working range, beam scan angle, and also to improve bar code symbol readability over a broad range of bar code densities.
Yamazaki et al., "New holographic technology for a compact POS scanner," Applied Optics, Vol. 29, No. 11 (Apr. 10, 1990) discloses a laser scanner in which the window of the scanner is replaced by a holographic plate, which is said to reduce the size of the system and thereby provide a more compact point-of-sale scanner to make the cashier's task easier.
It has also been known for a number of years to make use of a holographic disk as the scan element of a bar code scanner.